More than £5m is being spent on conserving one of Norfolk’s most-treasured historic buildings.

The National trust is spending £5.5m on repairing parts of the dormer windows, roof and chimney at Oxburgh Hall, near Downham Market.

A survey carried out after a dormer window collapsed in August 2016 led to “serious concerns” over the condition of parts of the 15th Century moated manor house,

It said death watch beetle, which caused the collapse, is present elsewhere in the building. New structural timbers are needed, while a “structurally questionable” chimney needs to be rebuilt.

The trust has applied to Breckland council for listed building consent to carry out the work.

A spokesman said: “Our roof tiles, timbers and brickwork are showing signs of aging at Oxburgh Hall, we’re not surprised, they’re well over 500 years old. So we’re currently planning a rather large conservation project that will begin in autumn 2019. The roof project, which will cost £5.5m, is likely to take around 18 months to complete.

“The National Trust has experience of carrying out conservation projects of this scale, with similar roof projects in recent years taking place at Dyrham Park, the Vyne and Castle Drogo, elsewhere in the country. Like those projects, Oxburgh Hall will remain open to visitors whilst the conservation work takes place, so visitors can find out more about the work that goes into caring for some of our nation’s special places.”

The trust’s planning statement says scaffolding will be put up around the building while work is in progress.

“The works are proposed following the collapse of one of the dormers,” it says. “Investigations into the cause of this collapse have identified the risk of a similar event occurring in relation to other dormer windows. During the surveys, other serious concerns have emerged regarding some elements of the roofscape and the Gatehouse. There is an urgent need to undertake further intrusive investigations, appropriate repairs, remedial and preventative works for the safety of visitors and staff and for the preservation of the historic fabric and significance of Oxburgh Hall.”